This invention relates to the use of Bernoulli pickups for the contactless handling of objects. In particular, the present invention relates to the use of such devices in the field of printing and more specifically, to the handling of printing plates.
There are many situations in which it is desirable to handle an object while eliminating or minimizing mechanical contact. This is particularly true when the physical condition of the surface of the object is a critical factor in the success or quality of an operation or product. Such situations are common in industries such as commercial printing, printed circuit board manufacture, and micro-lithography, which seek to make highly precise modifications to often delicate and mechanically sensitive surfaces.
In the printing industry, a typical example of a delicate and mechanically sensitive surface is the emulsion-coated imaging surface of a lithographic printing plate. Such plates typically comprise a substrate coated with one or more layers of radiation-sensitive material, frequently referred to as an emulsion. The coating of emulsion, particularly unexposed emulsion, is typically delicate and easily marred by mechanical contact. The marring, in turn, can produce undesirable artifacts in the final printed product. As a consequence, the printing industry is particularly concerned with methods to reduce the possibility of damage-inducing mechanical contact with the imaging surfaces of printing plates.
While damage to the imaging surface of a printing plate can occur at any point in the life of the plate, the surface is particularly vulnerable during operations requiring the movement of an individual plate from one location to another. This is especially true of imaging operations, during which an individual plate must typically be lifted from a stack of plates and then transported to or through the imaging device. This is not a trivial operation, as printing plates, though delicate, also tend to be quite large (50xe2x80x3xc3x9760xe2x80x3 is a commonly available size). Mar-free handling of plates is further complicated by the fact that the topmost and therefore most accessible surface presented by a plate or stack of plates is typically the imaging surface, making the most likely candidate for a handling surface also the least desirable one.
In automated or semi-automated systems for handling printing plates, a number of approaches are commonly used. One main approach is to use mechanical methods to transport plates. In order to minimize the possible damage caused by mechanical handling, prior art methods of this type have relied upon mechanisms to either reduce the amount of contact with the imaging surface or to reduce the mechanical impact imposed by such contact.
Methods to reduce contact between transport mechanisms and the imaging surface include employing gravity for at least part of the transport, handling plates by the edges only, or restricting contact as much as possible to the bottom (non-imaging surface) of the plate. Such methods do not completely eliminate the possibility of damage imposed by mechanical contact with plates, as plates may be warped, bent, scratched, or dented by gripping, dropping, or conveying operations. The imaging surface is particularly vulnerable during attempts to remove an individual plate from a stack of plates, given the aforementioned problem that the most accessible surface of a stacked plate is generally an imaging surface. In such situations, not only is the uppermost plate vulnerable to damage from mechanical impact, but also the imaging surface of plate immediately below it.
Methods to reduce the mechanical impact of the gripping and transport mechanisms include padding the plate-contacting portions of the mechanisms with soft or resilient materials, as well as keeping the amount of force used to perform these operations to acceptable minimums. None of these methods, however, eliminate the risks imposed by mechanical handling, as mechanical contact with the plates must still occur.
The other main approach is to use vacuum to move and hold printing plates. This approach is employed because vacuum methods generally do not require extensive mechanical handling of objects to be moved, and because vacuum cups (also referred to as suction cups) are often made of relatively soft and resilient materials in order to enhance the seal they make with a surface. However, picking up a plate using a vacuum still requires mechanical contact in order to establish the necessary seal, and the physical impingement of a vacuum cup can be just as effective in marring an imaging surface as the impact of a careless hand or a mechanical gripper.
Ideally, methods used to lift and transport printing plates should be contactless, eliminating the requirement for mechanical contact with the printing surface. Potential methods of this type include levitation using magnetic fields, electrostatic fields, or air bearings. However, the use of magnetic forces to lift plates imposes a requirement that the plates be susceptible to electromagnetic forces. This is a requirement that is not substantially met by many plate types, particularly those that employ non-metallic substrates such as rubber or polyester. Electrostatic levitation is similarly limited, given the requirement that the plate be able to hold an electrostatic charge. Unwanted electrostatic discharges may also pose a risk to the surface of the plate. Flotation of plates on air bearings, an example of which is disclosed in U.S. Pat. No. 5,798,825, is an attractive method. However, air bearings do not address the problem of lifting a plate from a stack without causing damage to imaging surfaces.
Another possible method for the contactless handling of lithographic plates comes from the semiconductor industry. This method, long known in that industry, employs the well-known Bernoulli principle in order to lift and hold delicate objects such as semiconductor wafers. Pickup devices that employ this method are commonly known as Bernoulli pickups.
Bernoulli pickups are essentially pressurized fluid devices that can be used to pick up objects by creating pressure differentials between the surface of an object and a surrounding fluid medium such as air. These manipulators utilize the Bernoulli effect by forcing a fluid, typically a gaseous fluid such as air, to flow under positive pressure between the surface of the pickup device and the object to be lifted. A schematic view of a typical Bernoulli pickup, well known in the prior art, is shown in FIG. 1. In FIG. 1, Bernoulli pickup 1 comprises a pickup head 3 coupled to a positive pressure pickup fluid source 5. The pickup fluid 7, typically a gas, flows through pickup head 3 via a shaft 9, emerging from the pickup head 3 via an orifice 11. When the pickup head 3 is positioned in close proximity to an object surface 13, pickup fluid 7 is forced to flow through the space 15 between a pickup face 17 and object surface 13. Arrows indicate the general direction of flow for pickup fluid 7.
The flow of pickup fluid 7 out of pickup head 3 toward the outer edges of pickup face 17 creates a low-pressure region 19 (shown schematically in FIG. 1) between pickup device 3 and object surface 13. This low-pressure region 19 is formed in accordance with the Bernoulli principle, as flowing pickup fluid 7 moves through space 15 at a higher velocity than the surrounding fluid medium. Region 19 within space 15 is thus at a lower pressure relative to the surrounding medium. The force resulting from this pressure differential will be normal to object surface 13. If sufficient to overcome opposing forces (such as gravity) acting on object 21, this force can be used to lift object 21. in this disclosure, the force on the object resulting from the action of the Bernoulli effect will be referred to as xe2x80x9cBernoulli liftxe2x80x9d.
Note that low-pressure region 19 does not necessarily extend to the limits of pickup face 17. This is because the velocity of the fluid 7 flowing through space 15 decreases as it flows toward the outer edges of the pickup face 17. Surrounding medium 25 will also have a tendency to flow toward low-pressure region 19, further slowing fluid 7. Thus, the pressure of the fluid 7 at the outer edges of the pickup face 17 will approach the pressure of the surrounding medium 25, and there may be portions at the periphery of pickup face 17 that do not contribute to generation of Bernoulli lift.
The magnitude of the Bernoulli lift generated depends on several factors, including the flow rate of the fluid being supplied by the positive pressure fluid source, the density of the fluid, the diameter of the pickup shaft and the pickup orifice, the proximity of the pickup surface relative to the object surface, and the pressure of the surrounding medium. Also important, though possibly not essential, is the extent to which the positive pressure fluid can maintain a pattern of laminar flow as it passes through the space between the pickup face and the object. Surface features that tend to disrupt laminar flow and promote turbulent flow, such as sharp protrusions or abrupt edges, will tend to reduce the amount of Bernoulli lift that can be generated with a given Bernoulli pickup.
Bernoulli pickups employing the basic mechanisms described above were suggested for use in the semiconductor industry during the 1960s and 70s. Patents such as U.S. Pat. No. 3,438,668 (Olsson et al.), U.S. Pat. No. 3,466,079 (Mammel), U.S. Pat. No. 3,523,706 (Logue), and publications such as xe2x80x9cWafer Pickup Headxe2x80x9d (Leoff et al., IBM Technical Disclosure Bulletin, January 1972), exemplify the basic approach. Examples of Bernoulli pickups for semiconductor handling can also be purchased; one example is the xe2x80x9cFloat Chuckxe2x80x9d distributed by Neumann Technology of 79 Loyang Way in Singapore. The SEZ group, a major supplier of semiconductor fabrication devices based in Villach, Austria, also employs the Bernoulli principle to ensure the contactless handling of thin semiconductor wafers (e.g., see U.S. Pat. No. 5,967,578, and the article xe2x80x9cSmart Card Assembly Requires Advanced Pre-Assembly Methodsxe2x80x9d, Semiconductor International, July 2000).
Aside from their use in the semiconductor industry, however, the use of Bernoulli pickups for contactless handling of materials is rare in industrial settings. A few patents, for example U.S. Pat. No. 3,880,297, U.S. Pat. No. 4,921,520, and U.S. Pat. No. 5,470,420 teach the use of Bernoulli pickups to handle other materials, such as sheets of glass and pressure sensitive labels. These methods, however, are not widely used. In particular, the use of Bernoulli pickups to move large and relatively heavy items such as lithographic printing plates is almost unknown as an industrial method, although U.S. Pat. No. 3,880,297 and U.S. Pat. No. 4,921,520 do propose such methods for moving glass plates. Neither of these last two patents, however, address crucial practical problems encountered when Bernoulli pickups are used to lift lithographic printing plates.
Part of the difficulty with using conventional Bernoulli pickups to handle printing plates is that conventional Bernoulli pickups are not well suited for handling very large sheets of delicately surfaced material. This is primarily because most of the existing art with respect to the use of Bernoulli pickups comes from the semiconductor industry, where the objects handled (semiconductor wafers) are generally much smaller than a typical lithographic printing plate. A large semiconductor wafer, for instance, might be up to 30 cm across; a large lithographic printing plate, on the other hand, may be as large as 130 to 170 cm across (50 to 60 inches). At present, design variations in Bernoulli pickups are generally directed toward (a) preventing lateral movement of objects, (b) decreasing the likelihood of contact with the object surfaces, (c) increasing the lifting capability of the Bernoulli pickup, and (d) stabilizing the object while it is being supported by the Bernoulli pickup. While improvements in Bernoulli pickup design from the semiconductor industry may also be applicable to the lifting of large lithographic printing plates, it is also the case that printing plates and similar objects present problems not typically encountered with semiconductor wafers.
The large surface areas of printing plates means that the pickups used to manipulate them will likely be considerably smaller than the plates. While it may be possible to construct a single Bernoulli pickup or an array of Bernoulli pickups large enough to cover the entire surface of the plate, the enormous pressurized fluid consumption required to maintain a low pressure zone over the entire surface area of a plate would make the operation of such an apparatus expensive and impractical. If only a few Bernoulli pickups are used to support a lithographic printing plate, it is quite likely that large portions of the plates will be unsupported. These unsupported portions will be free to flex and bend in response to forces such as gravity.
At the smaller scales required by semiconductor wafers, a primary concern is the restriction of the lateral movement of the wafer. As a result, many Bernoulli pickups for this industry employ sidewalls, extensions, prongs, or other measures to keep the wafer from sliding away from the pickup face. Examples of such features can be found in U.S. Pat. No. 4,969,676 and the IBM Technical Disclosure Bulletin entitled xe2x80x9cBeveled Bernoulli Headxe2x80x9d (Cunningham et al., September 1976). While such protrusions are suitable for handling objects whose perimeters fit within the boundaries defined by the protrusions, they are not suitable for handling large flat objects with delicate surfaces that extend beyond such protrusions, as the protrusions will not clear the edges of the objects during the operation of the Bernoulli pickups. Such protrusions, if present, can increase the risk of marring.
Printing plates, being substantially larger than typical semiconductor waters, also require more lifting force. An individual printing plate is also typically lifted from a stack of printing plates, so additional lifting force may also be necessary to separate an individual plate from the plate below it. Thus, while a lifting force equivalent to just a few grams may be sufficient to manipulate a semiconductor wafer, a lifting force equivalent to at least a half kilogram may be necessary to lift a printing plate.
For a given Bernoulli pickup, the additional force required to lift a heavier item can be generated by increasing the pressure differential between surrounding atmosphere and the space immediately between the pickup face and the object surface. The most obvious way to do this is to increase the velocity of the pickup fluid flow as it passes through the space between the pickup face and the surface of the object, for instance by increasing the pressure at which the positive pressure pickup fluid is fed to the Bernoulli pickup. This method is naturally limited by the pressure of the gas supply and related constraints, such as the need to limit the consumption of pressurized fluid. In many machines, the limitation of compressed air consumption by a particular process can be a vital consideration, particularly if the compressed air supply is necessary for several operations performed by the same machine.
Another possible method of increasing lifting force is to design the Bernoulli pickup face to incorporate additional features that encourage the formation of additional low pressure zones. Akashi (U.S. Pat. No. 5,067,762) and LaMagna (U.S. Pat. No. 4,969,676) both teach the use of a cavity in the pickup face in the area immediately surrounding the exit orifice of the positive pressure gas. Such cavities are reported to improve the stability of semiconductor wafers lifted with devices, possibly because the cavity helps to create a larger air cushion between the pickup and the wafer. It has also been suggested that the cavity becomes an additional zone of low pressure when the positive pressure fluid increases in speed as it leaves the cavity and passes through the relatively narrow space between the Bernoulli pickup face and the object surface. However, it is not clear whether such cavities do in fact increase lifting force, as the gains produced may be offset by losses in pickup fluid velocity within such cavities. The incorporation of such features also makes the three dimensional shape of a Bernoulli pickup more complex, reducing ease of manufacture.
Attempts to use conventional Bernoulli pickups with large, flat, thin objects such as lithographic printing plates also reveal an additional problem, one unlikely to have been encountered within the semiconductor industry. Prior art Bernoulli pickups, with pickup surfaces similar to that depicted in FIG. 1, produce high frequency, high intensity (approximately 110 db) sonic emissions when used to pick the large thin lithographic printing plates, whose base substrates are commonly made of aluminum. These emissions arise as a result of interactions between the conventional Bernoulli pickups, the flow of pickup fluid, and the plates themselves. The large, thin, elastically deformable surfaces of the plates function as huge resonators, maintaining and amplifying sound waves to unacceptable, ear-damaging intensities. This particular shortcoming is one that appears not to have been encountered in prior art uses of Bernoulli pickups, which is perhaps not surprising given that most of the art is directed at the manipulation of small, fragile objects such as semiconductor wafers. Unfortunately, it is also the case that prior art methods and apparatus do not address this particular problem.
The use of Bernoulli pickups for lifting a large, flat object such as a lithographic printing plate thus creates a set of technical requirements not typically encountered within the context of the semiconductor industry. First, it is necessary to ensure that the methods used when employing Bernoulli pickups for large objects will produce sufficient lifting forces. Second, marring of the surface by the Bernoulli pickup may be more likely than it would be for an object that is small enough to fit underneath the pickup, particularly if the Bernoulli pickup has protrusions on or near its face. Third, the larger forces required to lift objects of printing plate size tend to translate into higher requirements for pressurized fluid flowxe2x80x94that is, Bernoulli pickup systems for lifting large objects can be expected to consume much more pressurized gas than systems designed for lifting small, lightweight objects. At the same time, pressurized gas consumption by a single process within a machine may need to be limited if pressurized gas is used to drive other processes in the machine. Fourth, unwanted sonic emissions produced by the interaction of Bernoulli pickups with lithographic printing plates must reduced or eliminated. All of these requirements must be taken into consideration when designing a method for contactless handling of large flat objects using Bernoulli pickups.
It is the object of the current invention to provide a method for the contactless handling of articles using pickup devices of the Bernoulli type. More particularly, it is an object of the current invention to provide a method for the contactless handling of lithographic printing plates. Further objects and advantages of the present invention will become apparent upon consideration of the drawings and the ensuing description.
FIG. 2C shows an isometric view of a pickup head according to an alternative embodiment of the invention which comprises bristles on the pickup surface.
FIG. 2D shows an isometric view of the pickup head of a device according to a further alternative embodiment of the invention which includes a plurality of orifices.
The present invention provides a method for the quiet, contactless handling of objects using pickup devices of the Bernoulli type. The method is particularly suitable for the contactless handling of items much larger than an individual pickup, for example, the handling of lithographic printing plates. According to the method of the present invention, a flow of fluid is established between the pickup face of the Bernoulli pickup and the surface of the object to be supported. The fluid is made to flow over a pickup surface at a velocity sufficient to produce a pressure differential between the flowing fluid and a surrounding fluid medium. In the preferred embodiment, Bernoulli lift is maximized by making a portion of the pickup surface as smooth and protrusion-free as possible, and by ensuring that the location and extent of smooth surface substantially coincides with the maximal lateral limits of the low-pressure zone between the pickup face and an opposing object surface. As the pickup fluid flows beyond the periphery of the low pressure zone, it flows over a vibration-attenuating surface, reducing unwanted vibrations in the object.